Offshore oil and gas production is often conducted from platforns secured to the ocean bottom. One important design constraint for such platforms is that there be no substantial dynamic amplification of the platform's response to storms (a combination of wind, waves, and currents). Typically, most of a storm's wave energy falls within the 9 to 16 second period range. Consequently, to minimize the impact of storms on offshore structures, designers have engineered two types of structures, Steel Piled Jackets (SPJs) and Compliant Towers (CTs). These concepts differ mainly in the way they avoid wave energy. The SPJ, which is a stiff battered (i.e., having sloped sides) structure, uses its 2 to 4 second natural period to remain substantially below the period of storm wave energy. On the other hand, the CT, a flexible unbattered structure, uses its 20-30 second natural period to stay substantially above the storm wave energy. Generally, SPJs are economically viable structures in water depths less than approximately 1,000 feet, whereas CTs are economically viable structures in water depths greater than approximately 1,000 feet.
In addition to storm loads, some platforms may also be subjected to earthquakes. As can be seen from FIG. 1, most earthquake (seismic) energy falls within the 0 to 2 second range. A conventional SPJ will likely have a natural period of 2 to 4 seconds. While this period is well distanced from the typical 9 to 16 second wave period of storm energy, it is close enough to the 0 to 2 second excitation period of earthquake energy so as to be heavily influenced. Typically, a SPJ, because its low natural period is close to the earthquake energy, will have a large response in a seismic event, and thus most members will be designed primarily by the earthquake loads. In areas where earthquake loads are strong, the resulting structure tends to be a relatively heavy, inefficient SPJ. A CT, on the other hand, would have little dynamic response during an earthquake, but would be a very expensive concept for developing compliancy in the water depths where an SPJ is typically the preferred alternative.
Therefore, it would be desirable to have a more efficient offshore structure which can be used in areas subjected to strong seismic activity. The present invention satisfies this need.